Hydraulic actuator for actuating a gas exchange valve of an internal combustion engine

ABSTRACT

The invention relates to a hydraulic actuator for actuating a gas exchange valve ( 1 ) of an internal combustion engine, which valve is especially closed by the effect of a spring. The actuator is configured by a feed pump ( 6 ) with intermittent and thus variable delivery. The feed pump ( 6 ) has a high frequency and can be equipped for this purpose with piezoelectric, magnetorestrictive and/or electrochemical actuators as the feed elements. Optionally, the feed pump ( 6 ) can be operated with a stop valve ( 7 ) that is disposed downstream of the gas exchange valve ( 1 ) and that is controlled or regulated depending on the feed pump.

CROSS REFERENCE TO RELATED APPLICATIONS

Applicant claims priority under 35 U.S.C. §119 of German Application No.101 13 722.2, filed: Mar. 21, 2001. Applicant also claims priority under35 U.S.C. §365 of PCT/DE02/00947, filed: Mar. 16, 2002. Theinternational application under PCT article 21 (2) was not published inEnglish.

This invention relates to a hydraulic actuator according to the preambleof claim 1 and a method of operating this actuator according to claim13.

Such an actuator is known from Swiss Patent 536 934 A.

A hydraulic actuator which operates according to the principle of ahydraulic pendulum is known from the SAE paper 960581 with the title“Camless Engine” by the authors Michael M. Schechter and Michael B.Levin, based on a lecture at the International Congress and Expositionin Detroit, Michigan on Feb. 26 through 29, 1996.

This invention is concerned with the problem of simplifying the designof a generic actuator and designing it so that variable, i.e., differentlift distances can be achieved easily and reliably in opening the gasexchange valve.

A generic actuator having the characterizing features of Patent claim 1provides a fundamental solution to this problem. An especially expedientmethod of operating such a device is the object of claim 13.

A piezoelectric-hydraulic actuating device for gas exchange valves ofinternal combustion engines with which different lift distances arefundamentally achievable when opening a gas exchange valve is alreadyknown from German Patent Application 198 39 732 A1, where piezo elementsact as displacement actuators on a hydraulic transmission system, i.e.,a uniform quantity of hydraulic fluid is displaced to actuate the valve.A piezo actuator according to Japanese Patent 5-20 27 08 A2 alsooperates in the same way with an engine valve system.

In comparison with the two piezo actuators mentioned last, which areknown as hydraulic actuators, the embodiment according to this inventionis based on the idea of not displacing a constant volume of a hydraulicfluid but instead delivering hydraulic fluid with a pump which operatesat a high frequency, and on operating the gas exchange valves with thequantity of hydraulic fluid delivered and/or actuating the gas exchangevalves as a function of the volume flow delivered. With the embodimentaccording to this invention, this yields the advantage in comparisonwith the drives known in the past that the device has a small design andis independent of the camshaft for actuation of the gas exchange valvesof an internal combustion engine. In the case of feed pumps with piezoactuators, a small design is obtained for example due to the fact thatthe number of piezo elements with which only a small extent isachievable in each case can remain small to achieve a sufficiently largedisplacement volume. In addition, with the solution according to thisinvention, no provisions need be taken to compensate for hydraulic fluidleakage losses because there is no hermetically sealed hydraulic systemaccording to this invention.

In comparison with the generic state of the art described above, anotheradvantage of this invention is that it is sufficient to have ahigh-pressure reservoir and a switching stop valve situated upstreamfrom the gas exchange valve, and furthermore a variable lift position ofthe gas exchange valve can be achieved easily and safely. In addition,the device according to this invention operates in a power-saving modebecause the feed pump cannot operate continuously but instead operatesonly intermittently to open or close the valve. The feed pump usuallyoperates to open the gas exchange valve, but its closing action takesplace under the force of a restoring spring. The gas exchange valvecould also essentially close by hydraulic means and open under the forceof a restoring spring. In addition, according to this invention, thevalve may also be operated according to the principle of a hydraulicpendulum.

It is important for the implementation of the teaching according to thisinvention that an accurately controllable and/or regulable feed pumpwhich operates at a high frequency is used. Suitable feed pumps of thistype according to this invention include in particular those havingpiezoelectric, magnetostrictive and/or electrochemical actuators asdelivery elements. The displacement volume of hydraulic fluid necessaryfor opening or closing a gas exchange valve against an opposing forcecan be generated in the circulation by the fact that the circulatingflow is entirely suppressed or at least dammed up during thehydraulically actuated opening or closing operation downstream from thegas exchange valve, thereby creating a great flow resistance downstream,so that opening or closing of the gas exchange valve can be accomplishedby overcoming this opposing force. The damming up may be accomplished bya throttled flow cross section. This cross section can be variedaccordingly for opening and closing the gas exchange valve of therespective function. However, it is also possible to leave the throttlecross section unchanged and to vary only the delivery volume of thehydraulic fluid in accordance with the function for opening and/orclosing the gas exchange valves. The hydraulic force acting on a gasexchange valve which is to be actuated thus depends on the differencebetween the volume flow upstream and downstream from the gas exchangevalve within the hydraulic fluid circuit during the same period of time.In the case of a hydraulic pressure p on the gas exchange valve, avolume flow Qv upstream from the gas exchange valve and a volume Qndownstream from the qas exchange valve, this yields the followingfunctional dependence for the hydraulic pressure available at the gasexchange valve for opening or closing it: p=f (Q_(v), Q_(n)).

Expedient embodiments of the actuator for gas exchange valves of aninternal combustion engine are the object of the subclaims.

In the case of opening and closing the gas exchange valve under ahydraulic force according to this invention, the respective opposingmovement of the gas exchange valve takes place under an opposing force,which may be generated by a mechanical spring in particular. Therespective return movement should be dampened if possible. To this end,a flow cross section located downstream from the gas exchange valvewithin the hydraulic fluid circuit may be throttled temporarily, i.e.,during the desired damping time. However, during the return movement ofthe gas exchange valve, it is also possible according to claim 13 for adamping force to be produced by a temporary increase in the deliveryvolume flow of hydraulic fluid with a uniform circulation flow crosssection, downstream from the gas exchange valve in particular. In thisway, no control valve is necessary downstream from the gas exchangevalve in particular, i.e., if a valve is used there at all, a simplecut-off valve is sufficient.

The device according to this invention also permits in particularactuation of the gas exchange valves in a braking operation of theinternal combustion engine.

Feed pumps having piezoelectric, magnetostrictive and/or electrochemicalactuators as delivery elements and optionally similarly actuated pumpvalves which can be used expediently and to advantage for the presentinvention are also suitable in the same way in particular as injectionpumps for internal combustion engines.

Exemplary embodiments of this invention on the basis of which theclaimed embodiments are explained in greater detail are illustrated inthe drawing.

The drawing shows in schematic diagrams:

FIG. 1 a an actuating device for a gas exchange valve with anintermittently operating high-frequency feed pump and a 2/2 switchingvalve as a stop valve;

FIG. 1 b a diagram showing the crankshaft angles KW of an internalcombustion engine plotted on the abscissa and the opening lift H of thegas exchange valve plotted on the coordinate to illustrate differentopening stroke lengths available with the device according to thisinvention;

FIG. 1 c an actuating device according to FIG. 1 a for several gasexchange valves;

FIG. 1 d an actuating device according to FIG. 1 a with a mechanicaltransmission;

FIG. 2 an actuating device according to FIG. 1 with a feed pump designedas a piezo pump;

FIG. 3 a device according to FIG. 2 with a regulating valve instead of a2/2 switching valve;

FIG. 4 a device according to FIG. 2 with a 2/2 switching valve, shownstructurally, and a displacement sensor on the gas exchange valve to beoperated in a first embodiment of the displacement sensor;

FIG. 5 a device according to FIG. 4 with a second embodiment of thedisplacement sensor;

FIG. 6 a device according to FIG. 2 with a switching device forreciprocal actuation of a plurality of gas exchange valves by a commonpiezo pump;

FIG. 7 a device according to FIG. 2 with a camshaft-actuated 2/2-wayvalve as a stop valve;

FIG. 8 a device according to FIG. 2 with an actuator which intervenesadditionally in the adjusting hydraulic system for braking operation ofthe internal combustion engine having the gas exchange valves.

The hydraulic actuator device diagramed schematically in FIG. 1 a iscomposed of the following elements.

A high-frequency, intermittently or continuously operable feed pump 6conveys hydraulic fluid in a circulation system. Hydraulic fluid isdrawn out of a storage container 8 and conveyed back into storagecontainer 8 through lines 11 and 4, and a stop valve designed as a2/2-way switching valve 7. On route between feed pump 6 and the 2/2switching valve, a gas exchange valve 1 is connected to the circulatingline via a hydraulic chamber 2 of the gas exchange valve 1. A change involume of hydraulic chamber 2 results in a proportional lift adjustmentof the gas exchange valve 1. The lift path is indicated with arrows H inFIG. 1 a. A mechanical spring 19 is provided for the return movement ofthe gas exchange valve 1 against a hydraulically actuated adjustment.

This actuating device functions as follows.

The gas exchange valve 1 is held in the closed position by spring 19 inthe absence of an opposing hydraulic force. To open gas exchange valve1, feed pump 6 conveys hydraulic fluid with a volume flow Q_(v) intoline 11 with the 2/2 switching valve closed. The hydraulic fluidconveyed thus penetrates into hydraulic chamber 2 of gas exchange valve1 and thus causes the opening of gas exchange valve 1. To close the gasexchange valve 1, the delivery operation of feed pump 8 [sic; 6] isinterrupted and the 2/2 switching valve is switched to continuous flow.Delivery operation is preferably interrupted by a shutdown of feed pump6 which corresponds to the valve closing time. Feed pump 6 must be ahigh-frequency pump that can be switched without delay in the shortestpossible intervals of time. In particular, feed pumps 6 havingpiezoelectric, magnetostrictive and/or electrochemical actuators as thedelivery elements are suitable for this purpose. In the examplesdescribed below, a piezo pump is used as feed pump 6. The hydraulicforce which may act on the gas exchange valve 1, depending on thedelivery operation of feed pump 6, is entered in FIG. 1 a as a functionp=f(Q_(v), Q_(n)), where Q_(v) is the delivery rate upstream from thegas exchange valve 1 and Qn is the delivery rate downstream from the gasexchange valve 1.

An important advantage of the invention described on the basis of theschematic diagram in FIG. 1 a is that between the hydraulic chamber 2 ofthe gas exchange valve 1 and the feed pump 6, neither a stop valve nor apressure reservoir is necessary between such a stop valve and the feedpump. In the known state of the art, such a stop valve must always beprovided with such a device, whereby the feed pump delivers medium intothe high-pressure reservoir in the closed position of that stop valve.When that stop valve is opened, hydraulic fluid is delivered essentiallyout of this high-pressure reservoir and into the hydraulic chamber ofthe gas exchange valve to achieve a rapid response.

Variable lift adjustments of the gas exchange valve 1 can be achievedthrough different delivery or flow rates Q_(v) of the feed pump 6 withthe actuation device according to this invention, as illustrated in FIG.1 a. This is implementable through different delivery times, deliveryvolumes and/or delivery rates of the pump. The variability which canthus be achieved in the lift paths of the gas exchange valve 1 isillustrated in the diagram according to FIG. 1 b.

FIG. 1 c shows a device according to this invention having two gasexchange valves 1 and 1′ in which the two gas exchange valves operateaccording to the same principle described above. Functionally identicalparts are provided with the same reference notation but with anadditional index prime (′). In addition, cut-off valves 28, 28′ are allthat is necessary upstream from the two gas exchange valves 1, 1′ to beable to subject gas exchange valves 1, 1′ to flow in alternation.

In the case of the device according to FIG. 1 d, which essentiallycorresponds to that according to FIG. 1 a, the gas exchange valve 1 isactuated by way of an intermediate lever 30 as a mechanical transmissionaid.

An actuation device that operates according to the schematic diagram inFIG. 1 a is illustrated in FIG. 2 with respect to a piezo pump as thefeed pump 6.

The design of the device according to FIG. 2 is described as followsbelow.

The shaft of the gas exchange valve 1 of an internal combustion engineis designed as a displacement piston 3 which engages displaceably inhydraulic chamber 2 on the end of the shaft facing away from thecombustion chamber of the engine. Hydraulic chamber 2 is connected tothe hydraulic line 4 which is in turn connected to a delivery chamber 9of the feed pump 6 which is designed as a piezo pump by way of a one-wayvalve through which the medium flows in the direction of hydraulicchamber 2 and on the other hand the hydraulic chamber is connected tothe supply container 8 for hydraulic fluid by way of the cut-off valvedesigned as a 2/2-way switching valve 7. The hydraulic line 4 with thedelivery chamber 9 of the piezo pump 7 is connected between theswitching valve 7 and the storage container 8 by way of a one-way valve10 through which the medium flows in the direction of delivery chamber9. The connecting line 11 having one-way valve 5 leading away from thehydraulic line 4 also leads into the delivery chamber 9 of the piezopump 6. Piezo pump 6 consists of a housing 12 in which are mountedseveral piezo elements 13, stacked in layers one above the other. In thedirection of expansion, these piezo elements 13 act on a displacementelement 14 which is designed like a piston and acts on the deliverychamber 9 of the piezo pump 6 and is displaceably driven by piezoelements 13 to accomplish the change in volume inside of deliverychamber 9. Delivery chamber 9 is sealed with respect to the space of thepiezo pump housing 12 by the displacement element 14. To prevent tensilestresses, piezo elements 13 which are stacked together are under aprestress by a spring 15 supported on the housing 12 of piezo pump 6.Additional prestressing measures are also possible.

Piezo elements 13 can be acted upon electrically to produce alongitudinal expansion.

The hydraulic chamber 2 with the displacement piston 3 of the shaft ofthe gas exchange valve 1 guided in it is connected to the hydraulic line4 in two places. One of these connections, namely an opening 16, is usedexclusively to fill the hydraulic chamber 2 in the embodiment accordingto FIG. 2, while a second opening 17 is used mainly for discharging thehydraulic chamber 2 and has a greater flow resistance than does opening16. Opening 17 is designed so that its flow resistance is variable dueto the displacement piston 3 which passes over this opening, namely suchthat the flow resistance is increased as the size of the hydraulicchamber 2 becomes smaller. The inflow opening 16 is designed as aone-way valve through which medium flows only in the direction of theinterior of hydraulic chamber 2. This valve function is achieved by aspring-loaded ball pressed against the opening 16 from the interior ofthe hydraulic chamber 2.

When the valve actuation device is inactive, valve 1 is held in theclosed position by a spring 19.

The device described above functions as described below.

The piezo pump 6 acts as a high-frequency pump under electric activationof the individual piezo elements 13, this high-frequency pump conveyinghydraulic fluid out of the storage container 8 through the one-way valve10 and the delivery chamber 9 and then the one-way valve 5 into thehydraulic chamber 2 through an oscillating movement of the displacementelement 14, thereby opening valve 1. The prerequisite for opening valve1 is a closed 2/2-way switching valve 7.

To close an open gas exchange valve 1, piezo pump 6 is switched toelectrically inactive while at the same time opening the 2/2-wayswitching valve 7. The hydraulic fluid, which is under pressure in thehydraulic chamber 2, can flow out through opening 17 and through theopened 2/2-way switching valve 7 into the hydraulic storage container 8so that gas exchange valve 1 is closed under the force of spring 19. Dueto an increase in the flow resistance inside of opening 17 of thehydraulic chamber 2, the displacement speed of the valve shaft inclosing the gas exchange valve 1 is reduced, so that striking of thevalve of the gas exchange valve 1 on the valve seat is prevented.

The 2/2-way switching valve 7 is controlled and/or regulated incombination with the electric activation of the piezo pump 6 so thatperiodic opening and closing of the gas exchange valve 1 can take placein a fully variable manner.

The embodiment of the device according to FIG. 3 differs from thataccording to FIG. 2 in that instead of a 2/2-way switching valve 7 anelectric volume flow control valve 20 is used. This volume flow controlvalve 20 permits a delay in the speed of discharging of hydraulicchamber 2 when an electric current is adjusted when gas exchange valve 1approaches its closed position. Therefore, it is not necessary toprovide an additional discharge opening 17 in the hydraulic chamber 2 incomparison with the design in FIG. 2, so that then the one-way functionof the hydraulic chamber inlet opening 16 must necessarily beeliminated.

A delay in the restoring movement of the displacement piston 3 may alsobe achieved even without a discharge opening having a variable crosssection or the use of a regulating valve if the feed pump 6, whichshould essentially be inactive during the restoring movement, isswitched to active delivery with a time control to build up acounter-pressure which has a damping effect.

In an embodiment of the device according to FIG. 2, the shaft of valve 1is connected to a displacement sensor 21 in FIG. 4. In addition, the2/2-way switching valve 7 is illustrated there in a structurallyconcrete embodiment.

The concrete structural embodiment of the 2/2-way switching valve 7there consists of an electromagnetically operable valve-switchingdevice.

Displacement sensor 21 is designed as an inductive displacement sensor.The 2/2-way switching valve 7 is actuated as a function of thedisplacement signals of displacement sensor 21. The displacement signalsmay also be used for controlling and/or regulating the feed pump 6.

The embodiment according to FIG. 5 differs from that according to FIG. 4only in a different type of displacement sensor, which is designed thereas an eddy current displacement sensor 22.

In the device according to FIG. 6, a piezo pump 6 actuates a pluralityof gas exchange valves 1 via a multi-way switching valve 23. Switchingvalve 23 is actuated by electromagnetic actuators 24 such that the twogas exchange valves 1 are each acted upon hydraulically to open or closethem. This switching valve 23, which is designed as a slide valve, mayalso be implemented with a piezoelectric design if required by theswitching dynamics.

The hydraulic fluid may be kept under pressure in hydraulic storage tank8, to which end FIG. 6 shows a hydraulic pump 25 acting on the interiorof storage tank 8. The compression pressure for the storage tank may ofcourse be derived from any desired pressure source which is alreadypresent for other reasons, for example, in a motor vehicle. Due to thepressure acting on the hydraulic fluid in the storage container 8, it ispossible to minimize the influence of temperature on the intakeperformance of piezo pump 6 in particular. When using a hydraulic pump25, leakage losses can be compensated easily.

FIG. 7 shows an embodiment in which a 2/2-way switching valve 27 whichserves as a stop valve is operated by a special camshaft 29, forexample. Such a camshaft-actuated operation of the 2/2-way switchingvalve 27 may be expedient in operation of a gas exchange valve 1, whichfunctions as a discharge valve, in an internal combustion engine whichis operated more frequently in braking operation of the engine in aknown manner than is the case in motor-drive operation.

In braking operation of the engine in which the gas exchange dischargevalves 1 are operated repeatedly in comparison with the drive motoroperation for an additional charging and decompression, the dischargevalves 1 must be opened while under pressure. In order not to expose thedelivery chamber 9 of piezo pump 6 to this elevated pressure, anadditional hydraulic actuator 26, e.g., camshaft-actuated, may beprovided for controlling the respective discharge valves 1 in enginebraking operation, as illustrated in FIG. 8. This hydraulic actuator 26is connected to the hydraulic chamber 2 in an area between the 2/2-wayswitching valve 7, 27 and/or the regulating valve 20, which is used asan alternative, and the access opening 16. Piezo pump 6 is actuatable insuch a way that it is always switched to inactive when the hydraulicactuator 26 is active for opening the valve 1.

A variable actuator according to this invention has the followingadvantages in particular.

-   a: Good system dynamics are obtained due to a highly dynamically    operating pump, e.g., a piezo pump and the pressure acting on the    hydraulic fluid storage container. A high-pressure reservoir is not    necessary. The intake performance of the feed pump is subject only    to minimal temperature effects. It is thus possible to use the    actuator according to this invention over the entire rotational    speed range of the internal combustion engine.-   b: Full variability of the opening of the gas exchange valves is    achieved such as:    -   continuous phase shifting of the valve elevation,    -   variable lift setting and valve opening times,    -   variable valve opening time and valve closing time,    -   cylinder shutdown.

This leads in turn to the following:

-   -   higher power and torque of the internal combustion engine,    -   low consumption,    -   reduced emissions.

-   c: The number of components is reduced, namely    -   a camshaft is not necessary,    -   hydraulic valve level equalizing elements are eliminated,    -   it is unnecessary to have a valve brake in a valve displacement        measurement.

-   d: There is a great flexibility in assembly of the components    according to this invention.

-   e: Only a low level of hydraulic noise is generated and power    consumption is low because the piezo pump operates only in phases    for opening the gas exchange valve.

-   f: A stop valve downstream from a gas exchange valve inside the    hydraulic fluid circuit is subject to reduced dynamic requirements    because of the variable operation of the piezo pump, i.e.,    discontinuously, so that this stop valve can be actuated    electromagnetically.

-   g: No additional stop valve is necessary in the hydraulic fluid    circuit upstream from the gas exchange valve; in a conventional    hydraulic valve control system, such an additional cut-off valve    must operate at a very high dynamic level.

-   h: A high regulating accuracy is possible when using a displacement    measurement of the gas exchange valve because interference    quantities such as hysteresis, operating frequency, leakage losses    and temperature can be compensated in this way.

1. An actuator for actuating the gas exchange valves of an internalcombustion engine, whereby the gas exchange valves are actuated byhydraulic fluid conveyed by a feed pump in an open circuit equipped witha storage tank, whereby the opening or closing of the gas exchangevalves is produced by actuating forces emanating from the hydraulicfluid flowing through the circuit and acting at different levels on therespective gas exchange valve to be actuated, downstream from therespective gas exchange valve, the flow cross section of the circuit isconstricted or even closed with respect to all areas upstream from thisgas exchange valve, at least during the duration of a hydraulicallyactuated opening or closing of the gas exchange valve, the hydraulicactuating force on the gas exchange valve is counteracted by a force, inparticular a spring force, characterized in that the feed pump (6) isdesigned for a delivery operation which operates intermittently andworks at a clock frequency above that of the opening and closingfrequency of the gas exchange valves.
 2. The actuator according to claim1, characterized in that a mechanical force and/or distance translator(30) is provided between the hydraulic force source and the respectivegas exchange valve (1) and cooperates directly with the adjustmentmechanism of the respective gas exchange valve (1).
 3. The actuatoraccording to claim 1, characterized in that the feed pump (6) isdesigned with piezoeletric, magnetostrictive and/or electrochemicalactuators (13) as delivery elements.
 4. The actuator according to claim1, characterized in that the feed pump (6) is turned off at least duringa portion of the period of time when there is no hydraulically actuatedopening or closing of a respective gas exchange valve (1).
 5. Theactuator according to claim 1, characterized in that a shaft of the gasexchange valve (1) is displaceably and tightly inserted into a valvehydraulic chamber (2), which is filled with hydraulic fluid and isconnected to the storage container (8) by a line (4) containing theconstrictable to closable circulating flow cross section, and it isdisplaced there to open and close the gas exchange valve (1) with anincrease or reduction in size of the hydraulic chamber (2) produced bythe hydraulic fluid, and a greater flow resistance is to be overcome bythe hydraulic fluid to reduce the size of the hydraulic chamber (2) incomparison with the increase in size of this hydraulic chamber (2). 6.The actuator according to claim 1, characterized in that theconstrictable to closable circulating flow cross section is designed asa cut-off valve (7) or a throttling location.
 7. The actuator accordingto claim 6, characterized in that the cut-off valve (7) is a controlvalve.
 8. The actuator according to claim 1, characterized in that adisplacement sensor (21, 22) is provided for determining thedisplacement path of the gas exchange valve (1), and the feed pump (6)is regulated as a function of the positions of the shaft of the gasexchange valve (1) as determined by this sensor (21, 22).
 9. Theactuator according to claim 1, characterized in that in the case of anactuator, a plurality of gas exchange valves (1) are operable by a feedpump (6) with the help of a multi-way stop valve (23) or a plurality ofindividual stop valves (7, 28).
 10. The actuator according to claim 1,characterized in that the interior of the hydraulic storage tank (8) isunder pressure.
 11. The actuator for controlling the gas exchange valvesof an internal combustion engine according to claim 1 for a driving andbraking operation of the internal combustion engine, whereby in brakingoperation of the engine, the outlet gas exchange valves execute anadditional opening/closing cycle during the engine charging and optionaldecompression procedure in comparison with the engine drive operation,characterized in that a hydraulic actuator (26) which operatesseparately from the feed pump (6) can act on the hydraulic fluid in thehydraulic chambers (2) of the outlet gas exchange valves (1) forcontrolling the additional opening/closing cycles of the outlet gasexchange valves (1) in the engine braking operation, while the feed pump(6) is switched to inactive during this actuation time.
 12. The actuatoraccording to claim 1, characterized in that a displacement sensor (21,22) is provided for determining the displacement path of the gasexchange valve (1), and the stop valve (7, 20) is regulated as afunction of the positions of the shaft of the gas exchange valve (1) asdetermined by this sensor (21, 22).
 13. A method of operating anactuator for actuating gas exchange valves of an internal combustionengine comprising the steps of: (a) providing an actuator wherein thegas exchange valves are actuated by hydraulic fluid conveyed by a feedpump in an open circuit equipped with a storage tank, wherein: theopening or closing of the gas exchange valves is produced by actuatingforces emanating from the hydraulic fluid flowing through the circuitand acting at different levels on the respective gas exchange valve tobe actuated; downstream from the respective gas exchange valve, the flowcross section of the circuit is constricted or closed with respect toall areas upstream from this gas exchange valve, at least during theduration of a hydraulically actuated opening or closing of the gasexchange valve; and the hydraulic actuating force on the gas exchangevalve is counteracted by a spring force; and (b) operating the feed pumpfor an intermittent delivery operation at a clock frequency above thatof the opening and closing frequency of the gas exchange valves; whereinthe feed pump produces a braking force which acts on the respective gasexchange valve during the period of time in which a respective gasexchange valve is closed through targeted delivery operation within thehydraulic fluid circulation.